Haven't personally been into MB joints, but have done several on smaller cars. Usually you can take them apart, clean them up, and inspect the balls and ball tracks to tell if they're really worn to the point of needing replacement. They look mysterious to the novice, but are not really that difficult. Any good repair book can step you through the process.

As far as replacements, definitely rebuilt! I suspect that the increase in the front wheel drive auto population of the last decade has spawned a HUGE c/v joint rebuild industry. I think they just re-grind the races and put in oversized balls. (I have a mental image of this minimum-wage person sorting through a 20-foot high pile of used axles, throwing them in bins: GKN, NTN, etc...) Point is, price is relatively low and quality high, because this is a well-established industry.